1. Field of the Invention
The present invention relates to a supercharger and, more specifically, to a mechanically driven screw supercharger for supercharging internal-combustion engines such as automotive engines, marine engines and industrial engines.
2. Description of the Prior Art
Japanese Patent Provisional Publication (Kokai) No. 51-37316 discloses a mechanically driven screw supercharger 10 shown in FIG. 6.
The mechanically driven screw supercharger 10 has a set of male and female screw rotors 3 and 4 which are driven for rotation through a driving pulley 5, a belt 6 and a driven pulley 7 by a crankshaft 2 to such air through a suction filter 8 and to supercharge an internal-combustion engine (hereinafter referred to simply as "engine") 1. The screw supercharger 10 employing the screw rotors 3 and 4 is of a displacement type, and the rotating speed of the screw rotors 3 and 4 is proportional to the rotating speed of the crankshaft 2 of the engine 1. Therefore, the screw supercharger 10 has an advantage in that the engine 1 can be supercharged at the start and during acceleration without any time lag, which is impossible when using an exhaust turbine supercharger (turbo supercharger) driven by the exhaust gas.
Furthermore, having an internal compressing mechanism, the screw supercharger 10 operates at a lower compression loss and at a higher adiabatic efficiency as compared with the mechanically driven Roots supercharger.
FIG. 7 shows a conventional screw compressor applicable to the supercharger for the aforesaid application. This screw compressor is provided with a set of meshing female and male screw rotors (hereinafter referred to simply as "rotors") 3 and 4 rotatably supported in a casing 9. The rotative power of a prime mover 101 is transmitted through a driving pulley 5, belts 6 and a driven pulley 7 to the rotors 3 and 4. The driven pulley 7 is mounted fixedly on an input shaft 35, i.e., the rotor shaft of the female rotor 4, projecting outside through the casing 9. In this compressor employing the pulleys 5 and 7 and the belts 6 for transmitting the rotative power of the prime mover 101 to the rotors 3 and 4, the power is transmitted through the frictional engagement of the belts 6 and the pulleys 5 and 7. Therefore, a considerably high tension is applied to the belts 6 to produce a frictional force of a necessary magnitude between the belts 6 and the pulley 5 and 7. Accordingly, the input shaft 13 is subjected to a bending stress applied thereto by the belts 6 in addition to a torsional stress, and hence the input shaft 13 must have a considerably large diameter to withstand such stresses. However, an increase in the size of the rotor shaft, and hence the size of the bearings, entails an increase in the size and weight of the compressor, an increase in mechanical power loss attributable to the use of large bearings, and a lowering of the upper limit of the rotating speed of the rotors.
Furthermore, in the compressor of an oil-free type having synchronizing gears 11 and 12 as shown in FIG. 7, the bending of the input shaft 13 spoils the setting of the compressor and is liable to cause the rotors 3, 4 to interfere with each other. Still further, it is difficult to combine this known screw compressor with an engine to use the same as a supercharger, because this screw compressor is intended for use as a compressor and not as a supercharger and, when used as a supercharger, the characteristics of the screw compressor need to match those of the engine.
As is obvious from the air demand characteristics of an engine 1 as shown in FIG. 8, air demand Q2 per combustion cycle, namely, the quantity of air to be sucked into the combustion chamber for one combustion cycle to maintain the output torque of the engine 1 at a fixed level, is substantially constant regardless of the engine speed, and hence air demand Q1 per unit time, namely, the quantity of air to be supplied into the combustion chamber per unit time, increases in proportion to the engine speed. However, supercharging pressure needs to be increased with engine speed to maintain the air demand Q2 per unit time, because rheological resistance and the resistance of the suction valve against the flow of air increase with engine speed.
The supercharger 10 has the above-mentioned excellent characteristics, while the inherent internal pressure ratio of the supercharger is invariable. Accordingly, when the pressure downstream of the supercharger 10, namely, the supercharging pressure, varies, there arise unavoidably in the supercharger of the prior art two problems, namely, a problem in that compression loss increases thus entailing an increase in the power consumption rate, and a problem in that the pulsative flow of air due to a the sharp pressure variation of the discharged air attributable to pressure difference between the inside and outside of the discharge port increases the noise of the supercharger 10.